The use of mobile communications networks has increased over the last decade. Operators of the mobile communications networks have increased the number of base stations in order to meet an increased demand for service by users of the mobile communications networks. The operators of the mobile communications network wish to purchase components for the base stations at a lower price and also wish to reduce the running costs of the base station. Active antenna arrangements with Doherty Amplifiers have proven to meet these goals.
The Doherty amplifier is first known from U.S. Pat. No. 2,658,959 as an efficiency improved amplifier arrangement made of vacuum tubes for modulated signals. Since then the name Doherty amplifier has been recognized in the industry to refer to two parallel amplifier stages (vacuum tubes which were subsequently substituted by transistors), whereby a first amplifier stage operates in class AB mode and a second amplifier stage operates in class C mode. Usually the first stage is biased in such a way that the first amplifier stage linearly amplifies the input signal of the first amplifier stage from zero excitation to carrier level. The first amplifier stage is therefore also called carrier amplifier.
The second amplifier stage is biased in such a way that the second amplifier stage amplifies input signals above a certain threshold, i.e. input signals above the carrier level. Therefore the second amplifier stage is usually called peak amplifier. In order to improve load balancing, the input signals of both the first amplifier stage and the second amplifier stage are shifted in phase so that the phase difference between the input signals of the first amplifier stage and the second amplifier stage is 90 degrees apart. In this way the phase of the output signals of the carrier amplifier (first amplifier stage) and the peak stage (second amplifier stage) are also 90 degrees apart. In order to form the output signal of the Doherty amplifier the phase shifted output signals are recombined in-phase.
In the original U.S. Pat. No. 2,658,959 the phase shifting between the input signal of the carrier amplifier and the input signal of the peak stage was achieved by a LC-voltage divider located between the input of the amplifier arrangement and the input of the carrier amplifier; and a LC-voltage divider located between the input of the carrier amplifier and the input of the peak amplifier. As a result of this arrangement one input signal was retarded by 45 degrees in relation to the input signal of the amplifier. In contrast hereto the input signal of the other amplifier was advanced 45 degrees in relation to the input signal. The overall effect was to make both the carrier amplifier and the peak amplifier work at a phase difference of 90 degrees.
In the paper “A New High-Efficiency Power Amplifier for Modulated Waves”, Bell Telephone System Technical Publications B-931 in 1936, Doherty also describes the use of 90 degree networks. FIG. 9 in this paper depicts two different applications. In the first application the input signal of the Doherty amplifier is passed through a −90 degree network before the input signal is fed to the input of the first amplifier stage, whereas the input signal of the Doherty amplifier is fed directly to the input of the second amplifier stage, without applying any phase changes. A second −90 degree network at the output of the second amplifier stage retards the output of the second amplifier stage, so that the output signal of first amplifier stage and the output signal of the second amplifier stage after the second −90 degree network are in-phase and can be re-combined. A second application depicted in the said figure shows how to use a negative shifting 90 degree network at the input and a positive 90 degree shifting network at the output of the same amplifier stage. By this arrangement the phase difference within the same amplifier stage is compensated and the signals of the first amplifier stage and the second amplifier stage can be re-combined in-phase.
The amplifier stages of a Doherty amplifier may be formed by bipolar transistors or field effect transistors (FET). US Patent Application Publication 2009/0179702 A1 shows a Doherty amplifier arrangement comprising first and second bipolar bipolar transistors, as well as first and second bipolar field effect transistors.
Further in Naratip Wongkomet, “A +31.5 dBm CMOS RF Doherty Power Amplifier for Wireless Communications”, IEEE Journal of solid-state circuits, vol. 41, 2006, pp 2852-2859, a differential Doherty amplifier in CMOS technology is described, that was designed for operate in the DCS-1800 band for both GSM and GSM/EDGE. The Doherty amplifier, presented in this paper, consists of a main signal path and a auxiliary signal path. The main signal path has an impedance inverter network, which gives 90° phase shift at the output. To equalize the delay of the two signal paths, a polyphase circuit is used as a phase shifter network at the inputs of the two amplifiers. The polyphase circuit is a RC-CR ladder which gives 90° phase difference at its two outputs. The main amplifier and the auxiliary amplifier both are differential amplifiers with differential inputs and differential outputs. The differential outputs of the main amplifier and the differential outputs of the auxiliary amplifier are fed into a symmetrical, passive impedance inversion network.
All components are integrated on a single CMOS die except for a capacitor in the output matching networks and baluns. In order to facilitate high integration a lumped element pi network consisting of a first capacitor, a spiral inductor, and a second capacitor is used to provide for the impedance inversion function. An output matching network consists of a pair of bondwires and the off-chip capacitor to transform the 50-Ω load impedance to approximately 8-Ω differential.